Up and Down-conversion for Solar Cells

What is Up and Down-conversion for Solar Cells?

Up-conversion is the process by which two or more low energy (long wavelength) photons, which are transmitted by the photovoltaic cell, are converted into one high energy photon. The up converted photon has sufficient energy to bridge the solar cell band gap and can be absorbed. Down-conversion is the process by which one high energy photon, which is inefficiently absorbed by the photovoltaic cell due to thermalisation losses, is converted into two or more lower energy photons. These photons all have sufficient energy to be absorbed by the photovoltaic cell, with negligible thermalisation loss.

Up and down-conversion material is fabricated in layers which are independent of the photovoltaic cell. Therefore the optimised electrical properties of the cell are maintained. The up-conversion layer lies between the photovoltaic material and the back reflector, whereas the down-conversion layer lies on top of the photovoltaic material.

What are the challenges for Up and Down-conversion?

A significant challenge for up-conversion is that it is a non-linear optical process. This means that a very high intensity of incident light is required to produce only a small intensity of up-converted light. Furthermore, developing stable materials that exhibit high photon conversion efficiency is required.

The development of down conversion materials that exhibit the required energy conversion is the first challenge to be overcome. Once achieved, the down conversion layer will sit on top of the photovoltaic material: therefore the down-conversion layer must not interact with the light from the sun that is normally absorbed. A further challenge of down-conversion is to limit the loss of down-converted photons from the top surface of the layer.

How are these challenges being addressed by SISER researchers?

Heriot Watt University is carrying out research into the enhancement of up- and down-conversion layers for photovoltaic applications. Their work is based on the use of rare earth ions, which have been shown to be ideal candidates for the up-conversion process: they have luminescent properties that cover the required energy range (UV, visible and near-IR) and these properties are fairly invariant to the host material in which they are placed. Their current work is based on finding ways to control the propagation and interaction of light through these layers, to enhance the conversion. Heriot Watt University is working alongside the Fujian Institute of Research in the Structure of Matter, who are developing new luminescent materials for up and down-conversion.

The schematic diagram shows how up and down-conversion layers are applied to silicon solar cells. These layers are electronically isolated from the photovoltaic material, therefore do not experience the voltage matching problems associated with tandem solar cells. Please click on the image to expand.